1. Field of the Invention
This invention relates to stand-alone hydrostatic transmissions as well as combined hydrostatic and gear transmissions having housing structures provided with either independent or common sumps, such transmissions being usefully employed for many diverse applications such as vehicle drive lines of the type commonly referred to as hydrostatic transaxles.
This invention is particularly concerned with an improved hydrostatic transmission or transaxle drive line disposed within a surrounding housing structure and where the interior space inside the housing can be said to be divided by structural walls or bulkheads into two distinct internal volumes. The first internal volume containing the hydrostatic transmission submerged in its operating fluid whereas the second internal volume, being either in the form of a spill over chamber or alternatively, a chamber containing a gear train, are arranged to be fluidly linked together at all times by a communication duct in the form of a siphon.
2. Description of the Related Art
Hydrostatic transmissions and transaxles are increasingly being used in the lawn care industry and for other outdoor power equipment duties such as snow-blowing. They have become the preferred choice for power transmission drive lines; for example, in lawn and garden tractors with most employing a single hydraulic pump fluidly connected to a single hydraulic motor. Although in most instances single motor hydrostatic transmissions coupled by speed reduction gearing to a mechanical differential, applications also exist where two hydraulic motors are used and where each hydraulic motor is connected by a respective gear train to axle output shafts. Furthermore, two hydraulic pumps can also be used with two such hydraulic motors to create a hydrostatic transmission for each drive wheel which can be useful for zero-turn radius vehicle applications. Occasionally, single motor hydrostatic transmissions are used without the addition of a mechanical differential, such that the hydraulic motor is coupled by speed reduction gearing to a single output shaft, and in these instances, the output shaft may be the axle driving one wheel of the vehicle or be arranged to drive the axle of the vehicle by an interconnecting chain drive.
All hydrostatic transmission require hydrostatic power transmission fluid in order to operate and the fluid acts as the medium to convey power between the pump and motor of the hydrostatic transmission. As the positive displacement fluid pumping mechanisms used by all hydrostatic transmissions and hydrostatic transaxles require careful and accurate manufacture to achieve the necessary close tolerance fits in order to minimize internal fluid leakage losses associated with high-pressure performance, a preferred practice is to prevent damaging contamination generated by general wear and tear in the power transmitting gear train from reaching the pressurised circuit of the hydrostatic transmission. By removing the chances for damaging particles of contamination from entering the hydrostatic pressurised circuit, especially important when sintered powder-metal gears are used in the gear train, a long and useful working life for the hydrostatic transmission can be expected.
Although by no means essential, it can nevertheless be desirable to position the hydrostatic mechanism in a fluid compartment which is physically separate from any adjacent compartments in which the gear train is located such that no exchange of fluid can take place and whereby damaging contamination in the gear train compartment remains confined to that compartment. Contamination containment by way of separate compartments is shown in U.S. Pat. No. 5,090,949 titled Variable Speed Transaxle, expressly incorporated herein by reference. Here a bulkhead is provided in the housing which carries a shaft seal, the shaft seal operating on the interconnecting drive shaft which mechanically couples the hydraulic motor of the hydrostatic transmission in the hydrostatic compartment to the first reduction gear of the gear train in the adjacent gear train compartment. As such, further quantifiable benefits are gained as the compartment providing the sump for the gear train need only contain the bare minimum quantity of oil to satisfy lubrication considerations. Thus by relying what in effect is xe2x80x9csplash lubricationxe2x80x9d, expense is saved as the quantity of fluid needed is less and the efficiency of power transmission is improved as the associated drag losses of the fluid contacting the rotating gears is much less then with a sump carrying a full capacity of oil.
On the other hand, with some hydrostatic transaxles, the hydrostatic transmission is arranged to operate within the very same oil bath as the speed reduction gearing (and mechanical differential when included) and such designs are commonly referred to as xe2x80x9ccommon sumpxe2x80x9d types. Typically, the gear train and the hydrostatic transmission lie adjacent one another at the same elevation and the oil level in the sump is kept near to the brim to ensure that the hydrostatic components remain properly submerged at all times and also to avoid any ingestion of air. With a gear train operating submerged in the oil bath, power losses are greater due to the increase in fluid friction associated with the wetted area in contact with the oil than would be the case with the xe2x80x9csplash lubricationxe2x80x9d types mentioned earlier. Such gear drag losses can be especially noticeable in winter time when the gears are required to revolve from rest in a sump in which the oil can be in an extremely viscous initial state, and the resulting higher than normal operational loads imposed on the components in the drive train are unavoidable. As it is not possible to select oils with different properties in the common sump design, a problem is posed as the optimum fluid type which would normally be selected as the preferred lubricant for a gearbox will have completely different characteristics as compared to the type of power transmission fluid most suited for the efficient operation of a hydrostatic transmission. Typically a gear oil tends to be thicker with a high viscosity range whereas an automatic transmission fluid (xe2x80x9cATFxe2x80x9d) tends to be much thinner with a lower viscosity curve. As the hydrostatic transmission normally prevails when a conflict in design arises, it is accepted that the gear train may be operated in a generally adverse environment of low viscosity fluid such that accelerated wear and resulting higher contamination levels are more likely. The common sump design has a further limitation in that grease cannot be employed as the lubricant for the gear train. For certain applications, grease can be a more economic choice of lubricant.
Under normal atmospheric conditions, hydraulic fluids contain about 9% by volume of dissolved air which has virtually no effect on the physical properties of the fluid and therefore does not lead to any reduction in the performance of the system. However, should any appreciable quantity of undissolved air be present, the fluid will be prone to foaming problems, especially should the fluid experience excessive agitation, for instance, by any revolving elements such as gears being operated in only a partially submerged condition in the fluid sump. If such foaming occurs, it will rapidly lead to the destruction of the hydrostatic transmission. It is also a physical characteristic of the fluid to expand and contract in volume in relation to changes in its temperature. In general terms, the volume of oil increases by about 0.7% for every increase in temperature of 10 deg. C. and as hydrostatic transaxles can operate at below sub-zero ambient temperatures as well as on occasion above 100 deg. C. oil temperature, it is necessary to include an additional dead space volume of about 8% to allow for such volume expansion to occur without restriction over its initially contracted volume state. Accordingly, the fluid level in the sump rises and falls in relation to such temperature variation.
Quite often, an external expansion tank has to be fitted to the transaxle housing to cater for such volume changes in the quantity of fluid held in the sump. Should the tank be vented to atmospheric and rely on gravity-fed to work, such an external expansion tank can be troublesome to include as it must then be situated directly above the transaxle itself. Frequently the space available under the frame of the vehicle is needed for rear-discharge ducts for the grass clippings, and therefore, little space remains between the chassis and the transaxle for an adequately sized header or expansion tank. Recent attempts to overcome this problem are disclosed in U.S. Pat. Nos. 6,073,443 and 6,185,936. Both patents show the use of a siphon to connect the internal chamber of a hydrostatic transaxle to an external tank, the siphon allowing the tank to be located to one side of the transaxle housing exterior and at an elevation below the fluid level in the internal chamber. Although this solution does overcome one problem, namely the lack of available height in the installation, such external tanks may be vunerable to being damaged, for instance by stones kicked up by the revolving grass mower blades puncturing the tank and allowing fluid to escape to the environment. Furthermore, during severe winter conditions, a start-up of the hydrostatic transmission in such conditions while fluid in the siphon is in a semi-frozen state may cause the rotary seals in the hydrostatic transaxle to blow out.
There therefore is a need for a new solution to overcome the above mentioned disadvantages, and in particular there would be an advantage if the volume change in the fluid held in hydrostatic transaxle could be accommodated in a more protected and heat insulated environment inside in oppose to outside the transaxle housing. Thus a solution whereby the external expansion/header tank could be entirely eliminated would have the additional advantage of reduced cost.
Although it has been known for the housing for the hydrostatic transaxle to be manufactured slightly larger than is strictly necessary in order to enable an additional space or void to be inclided near the top to cater for the expanded volume of fluid, such a solution is not always practical. However, this solution will work well so long as the air present in the void does not become mixed in with the oil before the oil has sufficiently warmed to expel, through a breather, the air pocket from the void. Such a breather vent or passage is normally positioned at the highest position in the housing, and allows the free flow of atmospheric air in either direction from the void such that the fluid level in the sump can rise and fall depending on the temperature condition of the oil. Even so, it is difficult to completely eliminate the chances for mixing of the air and the oil and the risk is ever present so long as the revolving componentry of the hydrostatic transmission, such as the input drive shaft to the hydraulic pump or the ring gear of the differential, are able to break through the surface of the fluid. In practice, as more oil has to be carried in a common sump transaxle as compared to a design having separate and distinct chambers for the hydro and gearing as mentioned earlier, a larger dead space volume has to be included to take care of the resulting increased volume expansion. Consequently as the oil warms up towards its normal operating temperature and before its expanded volume has yet to reach a maximum, the remaining void or space situated in close proximity with the highest positioned shaft or gear still contains some air, and as these revolving components break through the surface of the oil, the induced severe agitation is likely to led to air being pulled into the oil. Should such mixing occur to any great degree, it can be detrimental to the performance of the hydrostatic transmission as well as result in cavitation erosion on the load carrying bearing surfaces accompanied by pressure shocks and noises. The problem is further compounded should the undissolved air in the form of foam escape via the breather to pollute the environment.
A further problem can occur should the sump be insufficiently filled to the correct level of oil, as too low a level of oil can later cause the oil to aerate and foam when the transaxle is operated, whereas too much oil can result in it being expelled to the environment via the breather passage once it has expanded due to temperature rise.
A typical problem encountered with vertical input shaft machines, should the oil level be lower then specified, is premature failure of the related bearing or seal due to a lack of lubrication. Furthermore, such naturally vented aspirated hydrostatic transaxles once left to cool after use in humid atmospheric conditions, draw moist air through the breather as soon as the oil begins to contract in volume and often this results in mist in the form of condensation of water vapour forming on the walls of the sump. Such entrained moisture, if not at once expelled as steam by the hot oil when the transaxle is once more in use, can even in small quantities over a period of time accelerate sludging of the oil by forming emulsions and by promoting the coagulation of insolubles such as dust particles that are also drawn through the breather as particles of solid matter as the unit cools after use. In general, air entering the sump causes the gradual oxidation of the oil and this deterioration in the lubricating properties of the oil ultimately lowers the life span of the hydrostatic transmission. Such a deterioration in the quality of the fluid can be rectified by oil changes at regular service intervals, but to undertake this is both costly and complicated to do due to the nature of the construction of such transaxles.
There therefore would be an advantage to be able to take care of volume changes in the hydrostatic transaxle without either recourse to using an inconveniently positioned external expansion tank or by having to rely on an internal dead space void above the fluid with its attendant risk in the formation of foam. There would be further advantage if environmental airborne contaminants as well as moisture be entirely prevented from entering the first internal volume containing the hydrostatic transmission components, or at the very least be allowed to be first absorbed in the fluid contained in the second internal volume in order to thereby slow and impede their progress towards entering the first internal volume.
Hydrostatic transmissions tend to be quieter in operation and work more efficiently and effectively when the fluid within the low-pressure side of the closed-loop circuit is charged or boosted from an auxiliary pump. The addition of such an auxiliary pump increases the manufacturing cost of a hydrostatic transmission and often requires a higher power output from the engine in order to drive both the auxiliary pump and the main pump of the hydrostatic transmission. There would therefore be an advantage if the hydrostatic circuit could be pressurized without having to include an auxiliary pump.
It is one of the objects of this invention to create a positive head on the hydrostatic fluid entering the low-pressure passage of the hydrostatic transmission without recourse to using a charge pump. Preferably the spill over chamber or the compartment used to house the gear train is sealed from the environment, and a rise in pressure in the spill over chamber or gear compartment aided or induced by the expanding volume of fluid in the hydrostatic compartment produces a net increase of pressure experienced by the low-pressure passage of the hydrostatic transmission.
It is a further object of the invention to improve the running efficiency of the speed reduction gearing used in hydrostatic transaxles. To achieve this object, the surface level of lubricant in the gear sump is automatically adjusted in direct proportion to the operational temperature of the fluid contained within the hydrostatic chamber. Having initially a low level of lubricant in the gear sump on the one hand lessens the adverse effect of power-retarding drag losses, especially during cold weather winter operation, whereas on the other hand, a rising level of lubricant in the gear sump can ensure good lubrication even when temperatures are elevated and viscosity is low. It is therefore a still further object of the invention to enhance the operational characteristics for the hydrostatic transmission by performance matching with respect to the operation of the speed reduction assembly irrespective of the temperature conditions in the environment.
One aspect of this invention is to make better use of the interior space inside the housing and thereby attend to fluid volume changes due to fluid temperature variation, and as such, a portion of the interior space inside the housing serves as an overflow receiver for the hydrostatic fluid in the first internal volume. Catering for fluid volume change internally is a significant improvement over current transaxle practice, as traditional transaxle practice is to rely on external paraphernalia to achieve this end. External devices as such can be prone to leakage and it is therefore a further object of this invention to provide a new and novel solution whereby a fluid expansion chamber is incorporated internally rather than externally in a hydrostatic transmission or a hydrostatic transaxle.
As one example of the invention, an overflow receiver for the administration of volume changes in the first internal volume can be incorporated in a hydrostatic transmission of the stand-alone type. As often there are no gears needed in such stand-alone types, the overflow receiver as the second internal volume, is fluidly connected by the siphon to the first internal volume, so that expansion and contraction of fluid surrounding the hydrostatic transmission components can occur without restriction. The over-flow receiver may be vented to atmosphere or preferably, remains sealed such that fluid entering it from the first internal volume causes internal pressuization in the over-spill receiver as well as in the first internal volume and thereby enhancing the operational characteristics of the hydrostatic transmission.
As a further example of the invention, the overflow is in the form of the gear compartment sump.
In one form thereof, the invention is embodied as a hydrostatic and gear transmission having an integral or combined housing formation whereby the interior space provided by the housing formation can be said to comprise a first internal volume expressly used for the purpose of accommodating components comprising the hydrostatic transmission and a second internal volume expressly used for the purpose of accommodating components of the gear transmission. The first internal volume contains the hydrostatic transmission submerged in its operating fluid whereas the second internal volume provides a fluid sump to lubricate the speed reducing gearing. First and second internal volumes are arranged to be fluidly linked together at all times by a communication duct in the form of a siphon such that any change in the volume of the fluid held by the first internal volume due to temperature change is translated by a flow of fluid through the siphon to effect an equal but opposite volume change in the fluid held by the second internal volume. The gear compartment sump may be vented to atmosphere or preferably, remains sealed such that fluid entering produces internal pressuization of the first internal volume such that the operational characteristics of the hydrostatic transmission may be enhanced.
Regardless whether the second internal volume be so configured as to be able to accommodate the gear train or not, it is to be preferred that the first region should remain completely full of hydrostatic fluid at all times.
According to the invention from another aspect, the interior space inside the housing can be said to be divided by structural walls or bulkheads to form these two distinct internal volumes.
Since the overflow receiver serves to receive displaced fluid from the first internal volume containing the hydrostatic transmission, there is little possibility for fluid from the first internal volume to escape into the environment. It is also an object of the invention to provide a simple contamination trap juxtapose the open-end of the siphon duct in the second internal volume so to reduce the likelihood of contamination from being able to enter the first internal volume and damaging the hydrostatic transmission.
In the detailed description and drawings which follow, the internal fluid expansion chamber for a hydrostatic transmission is shown in one form for both the first and second embodiment.